The world’s only licensed vaccine for dengue may worsen subsequent infections if used in areas with low rates of the mosquito-born disease, a new study has warned.

The world’s only licensed vaccine for dengue may worsen subsequent infections if used in areas with low rates of the mosquito-born disease, a new study has warned.

These infections are also more likely to need hospitalisation, researchers said.

The study by scientists, including those from Imperial College London, analysed all publicly available clinical trial data for the vaccine.

The results suggest that in people who have never been exposed to dengue before, the vaccine primes the immune system so that if they are subsequently infected, the infection is more severe.

However in people who have been exposed to the virus before vaccination, the vaccine reduces the severity of future infections, researchers said.

The researchers recommend testing people before they receive the vaccine, to establish if they have previously been exposed to the dengue virus. This would help avoid triggering an increase in serious cases of the disease.

Dengue is a viral infection that causes just under 400 million cases per year. According to the latest estimates, around half of the world’s population are at risk.

Unlike most infectious diseases, the second time a person is infected with dengue is usually far more serious than the first. This may be why the vaccine appears to amplify the illness in some individuals, particularly young children.

Normally, when a person is infected with a virus their immune system builds defences against it. This means when they are infected a second time, the virus is destroyed before triggering symptoms.

However, with dengue, the virus primes the immune system to work against the body. So when a person is infected a second time, a component of the immune system called antibodies help the virus infect the cells, leading to a more severe infection.

“If someone has never been exposed to dengue, the vaccine seems to act like a silent infection,” said Professor Neil Ferguson, Director of the MRC Centre for Outbreak Analysis and Modelling at Imperial College London.

“The initial exposure to the virus from the vaccine primes the immune system, so when they are infected again, the symptoms are more likely to be severe,” said Ferguson.

The vaccine, produced by the company Sanofi-Pasteur, is available in six countries and has been trialled on around 30,000 people from ten countries.

After analysing the data, the research team formulated a computer model to predict the effectiveness of the vaccine if used more widely.

“Having a licensed dengue vaccine available is a significant step forward for dengue control. However, we should be careful in considering where and how to use this vaccine as there is still uncertainty about the impact,” Ferguson added.